This invention relates generally to the optical components used in optical communication networks, and specifically to an optical component that combines a tuning element with one or more diffraction elements to create a tunable optical filter.
Optical communication networks are built by combining sub-systems, modules, or components which perform specific functions, including the function of selecting or removing a particular wavelength or group of wavelengths from an optical signal that contains multiple wavelengths. A general description of optical networking functions and applications can be found in “Introduction to DWDM Technology”, by Stamatios Kartalopoulos, Wiley-Interscience, 2000. Traditionally, the function of selecting or removing a particular wavelength or group of wavelengths from a multiple-wavelength optical signal has been implemented using fixed optical filters, incorporated into devices such as Optical Add/Drop Multiplexers (OADMs). Reconfigurable Optical Add/Drop Multiplexers (ROADMs) are OADMs in which the particular wavelength(s) that are being selected or removed can be modified dynamically. This enables dynamic and rapid reconfiguration of optical communication networks.
There is therefore a need to provide tunable optical filters that allow the dynamic reconfiguration of the particular wavelength or group of wavelengths that will be selected or removed from a multiple-wavelength optical signal.
FIGS. 1, 2, 3 represent three embodiments of the prior art in tunable optical filters. The prior art will be described in detail with reference to the figures. Similar elements in these three figures are labeled using numerals in which the last two digits are the same.
In FIG. 1, 101 is the input collimator consisting of a fiber ferrule and a micro-lens of filter 100. It carries multiple wavelengths or optical channels, λ0 through λn. 102 is a rotating or tilting thin-film bandpass optical filter, which selectively passes the preferred wavelength channel λi. 103 is the output collimator, which carries λi. The shortcomings of the filter 100 are that the motor (not shown) that is typically used to rotate or tilt the thin-film optical filter along the direction of the arrow in FIG. 1 is bulky and slow. The mechanical wear-out mechanism of this motor is not a good fit to the multi-million cycles that are required by the network application. Also, the width of the passband of the filter varies as a function of the tilt angle, resulting in optical characteristics that are not uniform across the filter's tuning range. The polarization dependent loss (PDL) that results from the angled thin-film filter is also a concern.
In FIG. 2, 202 is a linearly-variable thin-film filter that is moved laterally across the optical beam by a stepping motor (not shown) along the direction of the arrow in FIG. 2. The passed wavelength changes continuously across the length of the linearly-variable thin-film filter. Thus the preferred wavelength λi is selected and passed. The major shortcomings of this prior art are similar to that of the prior art shown in FIG. 1, in that the required motor is bulky, slow, and prone to premature wear-out. In addition, it is very difficult to fabricate a linearly-variable thin-film filter that will separate the narrowly spaced optical channels that are used in modern DWDM systems (e.g. DWDM systems with 50 GHz. Channel spacing), due to thin-film non-uniformity across the beam area.
In FIG. 3, an electrically, thermally, or acoustically controlled tunable wavelength etalon device serves as the wavelength selecting element. By changing the effective length of the optical path through the etalon cavity, via the application of external energy, the appropriate wavelength can be selected. As an example, an electrically-controlled micromechanical etalon filter was described in the article entitled, “widely tunable Fabry-Perot filter using Ga(Al)As—AlOx deformable mirrors” in IEEE Photonics Technology Letters, pages 394-395, March, 1998. In this device, an electric voltage adjusts the air gap between two reflective layers to tune the central wavelength.
Other tunable filter vendors use one or more thermally-sensitive cavity layers between the reflective layers of a traditional thin-film filter structure. The cavity layer(s) have an index of refraction that is therefore sensitive to temperature. The central wavelength can therefore be tuned by causing the cavity layer's refractive index to change through temperature variation.
All of these thermally-actuated designs employ expensive actuation and/or control technology and are thermally unstable due to the intrinsic nature of the cavity material. Closed loop tracking and control mechanisms are usually needed, which increases cost and package size.
None of the above tunable optical filters is entirely satisfactory. It is thus desirable to provide tunable optical filters with improved characteristics.